CPU Restart Behavior in PCS 7: Warm Restart vs Cold Restart and Retaining Readback Setpoints

Cold Restart (OB102) is strictly prohibited in SIMATIC PCS 7 automation stations; only Warm Restart (OB100) is permitted. A Cold Restart resets all data blocks to their offline initial start values, wiping live operational setpoints, tuning parameters, and accumulated batch totals. To retain parameter values across restarts: always perform Warm Restart (OB100), execute CFC Readback ('Chart > Read Back') to save online parameters into the offline project database, and perform a program download so the CPU load memory contains the updated operational parameters.

Executive Summary

During maintenance and commissioning of SIMATIC PCS 7 (V8.0 through V9.1) automation systems, plant operators or maintenance personnel sometimes request a 'Cold Restart' (Kaltstart) of an S7-400 or CPU 410-5H controller, assuming it provides a fresh, clean system state. Engineers who perform CFC Readback prior to a Cold Restart are shocked to discover that setpoints, PID tuning values, and recipe parameters revert to factory defaults upon CPU startup, requiring an emergency program download. This article establishes the strict manufacturer rules governing S7-400 restart modes, explains memory retention mechanics, and provides the approved parameter retention workflow.

Architecture and Fundamental Mechanics

Memory Retention and Controller Restart Mechanics in SIMATIC PCS 7:

S7-400 / CPU 410 Restart Modes Explained:
Warm Restart (Warmstart - OB100):
- The ONLY Approved Restart Mode in PCS 7.
- Process: The CPU clears dynamic local stack data (VAR_TEMP) and non-retentive bit memories (M). However, all retentive Data Block (DB) data is preserved in work memory.
- Technological Impact: PID parameters, operator setpoints, alarm trip limits, motor runtime totals (CountOh), and current sequence states are retained identically. The plant restarts smoothly without process disruption.
Cold Restart (Kaltstart - OB102):
- STRICTLY PROHIBITED IN PCS 7.
- Process: The CPU reinitializes ALL data blocks (both retentive and non-retentive) back to their original offline default start values stored in load memory.
- Severe Process Hazard: Every operating setpoint entered by operators over months or years is wiped out. Control valves snap to initial positions, heaters revert to 0%, and batch recipes are corrupted, potentially causing plant trips or equipment damage.
Hot Restart (Wiederanlauf - OB101):
- Attempts to resume execution at the exact instruction interrupted during power failure. Not supported or permitted in standard PCS 7 architectures.
How CFC Readback Interacts with CPU Memory:
When an engineer executes Chart -> Read Back... in CFC, the ES queries online CPU work memory and copies current values into the offline project database on the engineering PC.
Crucial Principle: Readback changes files on the Engineering Station hard drive; it does not update the CPU's load memory (flash card/RAM)! If the CPU executes a Cold Restart, it pulls initial values from its own load memory, ignoring the offline readback files unless the engineer downloads the program to the CPU.

Step-by-Step Implementation Procedure

Step-by-Step Procedure to Safely Capture and Restore Operational Parameters:

Execute CFC Parameter Readback:
- Open the CFC editor in SIMATIC Manager.
- Select menu Chart -> Read Back....
- Select scope: All parameters (or Operator control and monitoring attributes S7_m_c).
- Execute readback. SIMATIC Manager reads current operational setpoints from CPU memory and saves them into the offline CFC chart database.
Update CPU Load Memory via Program Download:
- To ensure that even a catastrophic memory loss cannot revert setpoints, execute a program download from the ES:
- Select PLC -> Download -> Changes Only.
- Downloading writes the newly read-back setpoints into the CPU's load memory as the new baseline initial values.
Enforce Approved Restart Mode in HW Config:
- Open HW Config; double-click the CPU module properties.
- Select the Startup tab.
- Under 'Startup after POWER ON', set the radio button strictly to Warm restart - finished (OB100).
- Ensure 'Cold restart' is disabled.
- Save, compile, and download hardware configuration.
Execute Restart when Required:
- When restarting the CPU (via mode selector switch or engineering command), select Warm Restart exclusively.
- Observe startup execution: OB100 executes initialization logic, and cyclic execution (OB35) resumes with all operational values fully intact.

Verification

Verification Procedure:
1. Parameter Stability Audit: Record several operational setpoints (e.g. PID setpoints, level alarm thresholds) in WinCC Runtime. Trigger a CPU Warm Restart. Verify that every recorded setpoint remains identical after the CPU enters RUN.
2. Diagnostic Buffer Check: In STEP 7, open CPU Module Information -> Diagnostic Buffer. Confirm that the startup event logs Event ID 16#4301: Transition from STOP to RUN mode via Warm restart (OB100).
3. Offline-Online Consistency: Execute Options -> Compare Blocks in SIMATIC Manager; confirm zero discrepancies between offline DB values and online CPU memory.

Limitations and Operational Precautions

Operational Precautions and Process Safety Constraints:
- Never Execute Cold Restart on Live Plants: Inform operating and maintenance teams that Cold Restart is non-standard and dangerous in a PCS 7 DCS environment.
- Battery and Memory Backup: In classic S7-400 systems, maintain backup buffer batteries in power supply modules (PS 407/PS 405) to preserve work memory during power outages.
- CPU 410-5H Retention: The CPU 410-5H features internal non-volatile NVRAM that maintains retentive memory permanently without requiring backup batteries.

Engineer FAQ

Why did a Cold Restart reset all my operating setpoints to initial values?

Because a Cold Restart (OB102) reinitializes all data blocks back to their offline default start values stored in load memory, erasing retentive memory.

Which restart mode is approved for SIMATIC PCS 7 controllers?

Warm Restart (OB100) is the ONLY approved restart mode in PCS 7.

Does performing a CFC Readback protect values against a Cold Restart?

Not automatically. Readback saves parameters to the offline ES database; you must download the project to the CPU to update the CPU load memory start values.

Where is the default CPU restart mode configured?

In STEP 7 HW Config under CPU Properties -> Startup tab.

Does CPU 410-5H require backup batteries to retain setpoints during power failure?

No. The CPU 410-5H uses internal non-volatile NVRAM to retain data blocks permanently without batteries.

Technical References

Related Tags

Warm Restart
Cold Restart
CFC Readback
CPU Memory
Parameter Retention

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CPU Restart Behavior in PCS 7: Warm Restart vs Cold Restart and Retaining Readback Setpoints